Method for casting a mold

ABSTRACT

A method for casting a part, that includes the steps of: introducing a mold into a first housing; engaging the first housing with a second housing to define a second chamber; melting an ingot within the furnace; reducing pressure within the second chamber to a first predetermined pressure; pouring at least a portion of the melted ingot into the mold; adding an gas to the second chamber to raise the pressure to a second predetermined pressure; moving the mold such that it is engaged with the means for cooling; and solidifying the liquid metal within the mold.

BACKGROUND OF THE INVENTION

This invention relates generally to an apparatus and a method forcasting a mold and more particularly to an apparatus and a method forcasting a single crystal part, intermediate component, or mold.

Some applications of metals require components or parts that areexceptionally wear resistant and heat resistant. Among theseapplications are engines such as aircraft engines. Conventionally, suchdurable components are produced from single crystal components. Suchsingle crystal components are produced by precise cooling and controlledcrystallization of a molten metal within a mold. The crystallizationbegins at a seed crystal and proceeds in a predetermined directionrelative to the seed crystal.

One problem with such conventional methods of forming single crystalmetal parts is that the process must be conducted in an inertatmosphere. This reduces the flexibility available.

Another problem with conventional procedures and apparatuses for castingsingle-crystal parts is that it is difficult to optimize a coolinggradient for a single piece part such that a best or optimum caststructure can be achieved.

Another problem with such conventional methods and apparatuses is thatto be economically feasible many components must be produced per batchor run. This results in large equipment costs and process and materialcommitments.

Another problem is that the conventional methods don't work that wellwith molds for casting parts produced by additive manufacturing methods.

BRIEF DESCRIPTION OF THE INVENTION

At least one of these problems is addressed by an apparatus and methodthat can provide for exposure of portions of the apparatus to air duringthe process and utilizes a single chamber as a melt chamber and as amold chamber.

According to one aspect of the technology described herein, a method forcasting a mold, the method comprising the steps of: introducing a moldinto a first housing; engaging the first housing with a second housingto define a second chamber; melting an ingot within the furnace;reducing pressure within the second chamber to a first predeterminedpressure; pouring at least a portion of the melted ingot into the mold;adding an gas to the second chamber to raise the pressure to a secondpredetermined pressure; moving the mold such that it is engaged with themeans for cooling; and solidifying the liquid metal within the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figuresin which:

FIG. 1 is a schematic side elevation view of an exemplary investmentcasting apparatus positioned in a first configuration;

FIG. 2 is a schematic side elevation view of the apparatus of FIG. 1positioned in a second configuration; and

FIG. 3 is a schematic side elevation view of the apparatus of FIG. 1positioned in a third configuration.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 illustratesschematically an example of one type of suitable apparatus 10 forcarrying out an embodiment of a casting method disclosed herein. Theapparatus 10 provides for high temperature operation when open to theatmosphere and thus exposed to oxygen. Therefore a mold 14 can beintroduced into the furnace while both the mold 14 and the mold heater32 are hot. Therefore, the disclosed method supports a mold for castinga part (also sometimes referred to as a component, intermediate part, ormold) which is uniquely suited for an additively produced mold 14.According to the illustrated embodiment a single mold 14 is supported bythe disclosed method. It should be appreciated that in some embodiments,multiple molds 14 can be utilized.

The apparatus 10 includes a furnace 20, a handling system 50, and acooling apparatus 60. The furnace 20 is positioned near the coolingapparatus 60 such that the handling system 50 is positioned between thefurnace 20 and the cooling apparatus 60. The furnace 20 includes a firsthousing 21 that defines a first chamber 22. A plurality of viewports 23defined within walls of the housing 21 such that the first chamber 22can be viewed from multiple angles. An opening 25 is configured toprovide access to the first chamber 22 and is positioned in a bottomwall of the housing 21. Optionally, the opening 25 can be positioned inanother location that would provide suitable access and utility.

The first chamber 22 is configured to contain a melt heater 24 that is abottom pour skull melt heater in the illustrated embodiment and a moldheater 32. The mold heater 32 includes at least one high-power electricmolybdenum disilicide (MoSi2) heating element for high temperatures asis conventionally known. The mold heater 32 can be exposed to oxidizingagents such as oxygen.

A pressure sensor 29 is fluidly connected to the first chamber 22 suchthat it can monitor the pressure within the first chamber 22. Thepressure sensor 29 is electrically connected to a controller 18. A gasaccumulator 26 is also fluidly attached to the first chamber 22. The gasaccumulator 26 is configured to contain a gas to be used to purge thefirst chamber 22. The gas can be an inert gas such as argon. By way ofexample and not limitation, a suitable gas for purging can be one of thefollowing: argon, air, nitrogen, helium, hydrogen, and a combinationthereof. A temperature sensor 35 is positioned to monitor thetemperature of metal within the melt heater 24. The temperature sensor35 can be an optical or immersion thermocouple.

The controller 18 can be a general computer or a specialized controllerconfigured to operate the furnace 20, the handling system 50 and thecooling apparatus 60. The controller 18 is configured to receive inputsfrom the pressure sensor 29 and from the temperature sensor 35.

An ingot chute 28 is positioned to deliver ingots of metal to the meltheater 24 to be melted within the melt heater 24. By way of example andnot limitation, such ingots can be the following metals: aluminum,nickel, iron, cobalt, and a combination thereof.

The handling system 50 includes a lift apparatus 52 and an arm 55. Thearm 55 is movably connected to the lift apparatus 52 such that operationof the lift apparatus 52 causes movement of the arm 55. The liftapparatus 52 and the arm 55 can be configured as a rack and pinion orother such device known for controlled movement. A mold holder 56 ispositioned at a distal end of the arm 55 such that the mold holder 56moves with the arm 55. The mold holder 56 is configured to receive amold 14. The mold holder 56 could be water cooled depending on thecooling apparatus used.

The mold 14 can be a conventional mold for investment casting or can bea mold produced by procedures such as additive manufacturing.

Referring now to FIGS. 1, 2, and 3, the handling system 50 is configuredto move between a first position, a second position, and a thirdposition. In the first position, the mold holder 56 of the handlingsystem 50 is positioned outside of the furnace 20 and outside of thecooling apparatus 60 as shown in FIG. 1. In the second position, themold holder 56 of the handling system 50 is positioned within thefurnace 20 as shown in FIG. 2. In the third position, the mold holder 56of the handling system 50 is positioned within the cooling apparatus 60as shown in FIG. 3.

In this manner, the mold 14 can be loaded onto the handling system 50and then inserted into the furnace 20 via movement of the handlingsystem 50 between the first position and the second position such thatthe mold can be introduced into the furnace 20 in an initial step andcan be removed from the furnace 20 in a subsequent step. The handlingsystem 50 is positioned adjacent the furnace 20 and the coolingapparatus 60 such that the handling system 50 can transfer a mold 14into the furnace 20, from the furnace 20 to the cooling apparatus 60 andthen out of the cooling apparatus 60.

Referring now to FIG. 1, the cooling apparatus 60 includes a coolinghousing 61 which is configured to receive a vessel 62. A mixingpropeller 64 and a heater 66 are positioned within the vessel 62. Amixer 74 is configured to circulate a liquid metal bath contained withinthe vessel 62 in conjunction with mixing propeller 64. In theillustrated embodiment, the liquid metal bath is tin (Sn). By way ofexample and not limitation, the liquid metal bath can be the following:tin, aluminum, gallium, and a combination thereof.

The cooling housing 61 is a second housing and is supported by a lift68. The lift 68 is positioned to move between a lowered first positionand a raised second position such that the first housing 21 and thecooling housing 61 move relatively to each other. When the lift 68 is inthe first position, the cooling apparatus 60 is positioned apart fromthe furnace 20 as shown in FIG. 1. When the lift 68 is in the raisedsecond position, the cooling housing 61 is sealingly engaged with thehousing 21 of the furnace 20 as shown in FIGS. 2 and 3. Together thecooling housing 61 and the housing 21 define a large second chamber 82.The large chamber 82 is configured to function both as a melting chamberand a mold chamber in accordance with the method described below.

The lift 68 is configured to be controlled by the controller 18 incooperation with the lift apparatus 52 such that the mold holder 56 canbe introduced into and removed from the vessel 62. In this way, a mold14 containing molten metal can be introduced into the cooling apparatus60 and cooled via a means of cooling, and thus solidified, in acontrolled manner as will be described in more detail in the methodbelow. In the illustrated embodiment, the means of cooling is a liquidmetal bath, such as molten tin. It should be appreciated that thecooling apparatus 60 can be configured to utilize a broad range of moldcooling methods as the means for cooling. These methods can includeradiation cooling, forced gas cooling, liquid metal cooling, fluidizedbed cooling.

A dry vacuum pump 27 is electrically connected to the controller 18 andfluidly connected to the first chamber 22 and thus large chamber 82. Thepump 27 is configured to withdraw gases contained therein to create areduced pressure environment. It should be appreciated that types ofvacuum pumps other than dry vacuum pumps can be used as pump 27 in otherembodiments. The housing 21 is configured such that a hard vacuum can begenerated with the pump 27 within the first chamber 22. In someembodiments, the vacuum can be about 0.015 Torr. An example operatingpressure is 200 Torr such that the pressure within the first chamber 22can be determined by the controller 18 via the introduction orwithdrawal of gas and operation of the pump 27 to withdraw gases such asatmospheric gasses from the first chamber 22.

The presently disclosed technology can be better understood from adescription of the operation thereof. As indicated above, the apparatus10 is used in the investment process of casting. More specifically, theapparatus 10 is utilized to cast single crystal metallic components orintermediate structures that are directionally solidified in a moreefficient and simplified manner than conventional methods. It should beappreciated that the sequence of steps shown below can vary in differentembodiments as would be understood by one skilled in the art.

The disclosed technology provides a method for casting a mold, themethod includes the following steps: a loading step, a pressure reducingstep, a pouring step, a solidifying step, and an unloading step.

Referring now to the loading step, it includes the following steps: A)Preheating a mold 14 prior to introducing the mold 14 into the furnace20. More preferably, preheating the mold to greater than about 2200° F.In some embodiments, the mold 14 is preheated to about 2800° F. As usedherein, the term “preheating” refers to heating the mold 14 aboveambient temperature prior to introduction of the mold 14 into the moldheater 32. B) Placing the mold 14 on the mold holder 56. C) Operatingthe handling system 50. D) Introducing the mold 14 into the furnace 20such that the mold 14 is positioned within the mold heater 32. E)Operating lift 68 such that cooling housing 61 is engaged with housing21 thus defining large chamber 82. F) The mold heater 32 is operated tomaintain the mold 14 at a predetermined temperature. Preferably, thepredetermined temperature is about 2200° F. G) Loading an ingot viaingot chute 28 into the melt heater 24. Loading an ingot can be doneprior to, during, or just after the step of introducing the mold 14 intothe furnace.

Referring now to the pouring step, it includes the following steps: A)Reducing the pressure with the chamber 82 below atmospheric pressure byoperating the pump 27. B) Introducing a gas such as argon into the firstchamber 22 to replace the previous gasses within chamber 82. C) Reducingthe pressure within chamber 82 from atmospheric pressure to a purgepressure. The purge pressure may be in a range of from 100 to 300 Torror it may be in a range of 150 Torr to 250 Torr or it may be in a rangeof 190 Torr to 210 Torr and is preferably 200 Torr. As used herein, theterm “purge” refers to or is related to the process of replacing the gasor atmosphere previously within the second chamber 82. C) Melting theingot in the melt heater 24. D) Reducing pressure further to a pressurefor pouring. The pressure for pouring can be from 0.005 Torr to 0.05Torr or the pressure for pouring can be from 0.01 Torr to 0.03 Torr orthe pressure for pouring can be from 0.0125 Torr to 0.0175 Torr or thepressure for pouring can be 0.015 Torr. The pressure for pouring ischosen to minimize gas bubbles within the molten metal and to aid infilling the mold 14 with the molten metal. E) Pouring the molten metalfrom the melt heater 24 into the mold 14.

Referring now to the solidifying step, it includes the following steps:A) Introducing gas from accumulator 26 to maintain a pressure within thechamber 82 at a generally constant pressure. Preferably, the pressure isabout 200 Torr. C) Removing the mold 14 from the mold heater 32 byoperating the handling system 50. D) Engaging the mold 14 within themeans of cooling. Thus in the illustrated embodiment the mold 14 ispositioned within the vessel 62 such that it is surrounded by the liquidmetal bath. D) Keeping the mold 14 engaged with the means of cooling fora predetermined time. Preferably the predetermined time is between about2 minutes and about 30 minutes, more preferably the predetermined timeis between about 10 minutes and about 20 minutes and even morepreferably the predetermined time is about 15 minutes.

Referring now to the unloading step, it includes the step of operatingthe lift 68 such that the cooling apparatus 60 is disengaged from thefurnace 20 and the mold 14 is positioned between the furnace 20 and thecooling apparatus 60 such that it can be removed. The process can thenbe repeated with a new mold 14.

The technical advantages of the disclosed method and apparatus includeoptimization of cooling gradient to achieve the best cast structure.Such a cast structure would include less elemental segregation, crystaldefects and porosity. Another technical advantage is that a bottom pourskull melt can be utilized with the disclosed technology therebyachieving better cleanliness and chemical stability. Another technicaladvantage is that the mold can be loaded directly from a fire or preheatapparatus thereby minimizing thermal cycles. Thus a printed, firedand/or preheated mold can be loaded directly into the furnace. Anothertechnical advantage is improved fill consistency from gas backfillwithin cycle.

The commercial advantages associated with the disclosed technologyinclude low cost support for casting with directional solidification ina manner uniquely suited for additive manufacturing or 3D printingproduced molds. The disclosed technology allows for a mold produced by a3D printing or additive manufacturing method to be loaded directly intoa hot furnace in a preheated state.

The foregoing has described a method and apparatus for casting a mold.All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

What is claimed is:
 1. A method for casting a part, the methodcomprising the steps of: introducing a mold into a first housing;engaging the first housing with a second housing to define a secondchamber; melting an ingot within the second chamber; reducing pressurewithin the second chamber to a first predetermined pressure; pouring atleast a portion of the melted ingot into the mold; adding a gas to thesecond chamber to raise the pressure to a second predetermined pressure;moving the mold such that it is engaged with the means for cooling; andsolidifying the liquid metal within the mold.
 2. The method according toclaim 1, wherein the mold is preheated prior to the introducing step. 3.The method according to claim 2 wherein the mold is preheated to atemperature greater than 2200° F.
 4. The method according to claim 3,wherein the mold is preheated to a temperature of about 2200° F.
 5. Themethod according to claim 1 further comprising the step of: placing themold into a mold heater that is positioned with the first housing. 6.The method according to claim 5 wherein the second housing includes ameans for cooling.
 7. The method according to claim 1, wherein the stepof moving the mold such that it is engaged with the cooling meansincludes the step of operating a handling system.
 8. The methodaccording to claim 1, wherein the cooling means includes a liquid metalbath.
 9. The method according to claim 8, wherein the liquid metal istin.
 10. The method according to claim 1, wherein the cooling means isradiation cooling.
 11. The method according to claim 1, wherein thecooling means is forced gas cooling.
 12. The method according to claim1, wherein the cooling means is fluidized bed cooling.
 13. The methodaccording to claim 1, wherein the step of reducing pressure within thesecond chamber to a first predetermined pressure is preceded by a stepof reducing the pressure within the second chamber to a thirdpredetermined pressure.
 14. The method according to claim 13, whereinthe step of introducing a gas occurs before the pressure has reached thethird predetermined pressure.
 15. The method according to claim 14,wherein the gas is argon.
 16. The method according to claim 14, whereinthe step of introducing the gas comprises a step of replacing at least aportion of an atmosphere within the second chamber.
 17. The methodaccording to claim 13, wherein the third predetermined pressure is about200 Torr.
 18. The method according to claim 17, wherein the firstpredetermined pressure is less than 10 Torr.
 19. The method according toclaim 18, wherein the first predetermined pressure is about 0.015 Torr.20. The method according to claim 1, wherein the second predeterminedpressure is about 200 Torr.
 21. The method according to claim 20,wherein the gas is argon.
 22. The method according to claim 1, whereinthe step of solidifying metal within the mold includes keeping the moldin the means for cooling the mold for a predetermined time.